Buenos Aires
— In what may prove to be a significant breakthrough in UN talks on the Falklands dispute, Argentina is making a ''slight shift'' in its negotiating stance.

Argentina now says that negotiations with Britain over their future must lead ''inexorably'' toward sovereignty. Earlier it demanded Argentine sovereignty over the Falkland Islands must be recognized and accepted now.

Lt. Gen. Leopoldo Fortunato Galtieri, head of the ruling military junta, said early Thursday that Argentine sovereignty over the islands ''is, was, and will be the objective of the people of Argentina. We are not going to renounce this objective; we are going to talk about achieving it in a reasonable period.''

The shift, conveyed Wednesday to United Nations Secretary-General Javier Perez de Cuellar, represents a significant concession by Argentina, although it is not being called a concession here. It is one that all along, even through last weekend, was rejected outright by Argentine spokesmen.

But fresh diplomatic and military assessments -- which suggest that Argentina may not be as strong in these areas as the junta thought it was last month -- have signaled this change of stance on the sovereignty issue.

How much flexibility has been infused into the UN talks by the shift remains to be seen. But well-placed Argentine sources indicate that the new position is just about as far as the present military junta can go.

Behind the ''slight shift,'' as a Foreign Ministry official termed it, were enormous pressures on the military junta from both civilian and military circles.

It is understood that Argentine Foreign Minister Nicanor Costa Mendez recommended the change -- and fought for it tenaciously in meetings with the military junta headed by General Galtieri.

So did a number of General Galtieri's military associates, including both Army and Air Force generals. They argued that unless there was a give in the Argentine position, the military would be dragged further into war with Britain -- with most uncertain consequences for Argentina and for its military.

Ironically, sources say, the Argentine Navy is said to have opposed the shift in meetings as late as Tuesday. The irony comes in the fact that the Army and Air Force are as yet largely untested in combat, while the Navy has been badly battered in the past two weeks. The Navy now is virtually restricted to Navy bases and coastal waters due to British control of the seas in the South Atlantic.

New assessments of the relative military positions of Argentina and Britain have led key elements in the Argentine military to doubt that their Army and Air Force would emerge fully triumphant in an out-and-out battle with Britain.

These assessments say that Argentina may not necessarily be completely defeated in such engagements, but that the country could emerge a shattered nation from its first war in more than 100 years.

As a result, some generals are vigorously arguing for less sweeping objectives on the diplomatic front -- apparently holding out for the Costa Mendez position of an ''inexorable'' movement toward Argentine sovereignty.

The shift is said to result largely from Mr. Costa Mendez's initiative -- an indication of his own strength with the generals and a repudiation of speculation that he might be on the way out.

Virtually everyone in Argentina argues that ''the Malvinas (Argentina's name for the Falklands) are ours.'' But not everyone approves of the military's seizure of the islands April 2. The move shocked some Foreign Ministry officials. But once done they joined the military, arguing that Argentina possessed irrevocable, irrefutable sovereignty.

Although Argentina appears to have shifted its negotiating position, there is no shift in its view of the ultimate ''fact'' -- as sovereignty was termed by a Foreign Ministry spokesman here Wednesday.

But there obviously has been a shift in the time by which Argentina expects its sovereignty to be recognized -- not that a specific timetable has as yet been set up. That is left to the negotiations at the UN.

By making this diplomatic shift, Argentina believes the ball has been thrown into Britain's court. British Foreign Secretary Francis Pym, speaking in London, is quoted here as saying Argentina's new proposals are being studied carefully.

In the Argentine Foreign Ministry's view, there is likely now to be a great deal of jockeying. Argentina may well have to make some specific concessions within this shift on the sovereignty issue.